Apparatus for detecting positional errors utilizing high-frequency modulation of light source and phasesensitive detector

ABSTRACT

Apparatus is disclosed for detecting error in the positioning of a member to which the apparatus is attached in relation to a reference surface having a different reflectivity from the surrounding area. An extended light source modulated at a frequency high relative to mains frequency is adapted to produce dispersive illumination for floodlighting the reference surface and at least two photoelectrically sensitive devices are arranged for viewing substantially only light reflected from respective discrete areas on the reference surface spaced in one direction and for producing in cooperation with the reference surface output electrical signals representative of the illumination received by the devices. Means are provided for deriving a position error signal depending on the differences between the output signals.

States Patent Wilkinson et a1.

APPARATUS FOR DETECTING POSTTTONAL ERRORS UTILIZING HIGH-FREQUENCY MODULATION OF LTGHT SOURCE AND PHASE- SENSKTIVE DETECTOR Inventors: Alan Wilkinson, Egham; Stewart Charles Hine, Harrow; David Robert Bosher, Hayes, all of England Assignee: Electric 8: Musical Industries Limited,

Middlesex, England Filed: Nov. 28, 1969 Appl. No.: 880,615

Foreign Application Priority Data Sept. 9, 1969 Great Britain ..44,390/69 U.S. Cl. ..250/209, 250/209, 250/217 R,

250/219 DR, 250/220 M Int. Cl ..G0ln 21/30, G02! l/28, HOlj 39/12 Field of Search ..250/202, 219 W], 217, 209,

[56} References Cited UNITED STATES PATENTS 3,283,162 11/1966 Quittner ..250/217 X 3,427,457 2/ 1969 Domenico ..250/202 Primary Examiner-Archie R. Borchelt Assistant Examiner--T. N. Grigsby Attorney-William W. Downing, Jr.

[57] ABSTRACT Apparatus is disclosed for detecting error in the positioning of a member to which the apparatus is attached in relation to a reference surface having a different reflectivity from the surrounding area. An extended light source modulated at a frequency high relative to mains frequency is adapted to produce dispersive illumination for floodlighting the reference surface and at least two photoelectrically sensitive devices are arranged for viewing substantially only light reflected from respective discrete areas on the reference surface spaced in one direction and for producing in cooperation with the reference surface output electrical signals representative of the illumination received by the devices. Means are provided for deriving a position error signal depending on the differences between the output signals.

3 Claims, 4 Drawing Figures PATENTEUFEB 2 2 I972 SHEET 2 BF 2 APPARATUS FOR DETECTING POSITIONAL ERRORS UTILIZING HIGH-FREQUENCY MODULATION OF LIGHT SOURCE AND PHASE-SENSITIVE DETECTOR The present invention relates to apparatus for detecting positional errors and especially, although not exclusively, to apparatus for detecting positional errors of so-called stacker cranes in warehouses.

The storage racks used in warehouses commonly comprise a storage frame consisting of a number of horizontal and vertical members which form a two-dimensional matrix of horizontally and vertically aligned open-ended storage bays into which loads or load-bearing pallets may be inserted, or from which loads or load-bearing pallets may be removed, by means of stacker cranes which transport the loads or pallets. A stacker crane consists basically of a vertical mast which is driven horizontally along a rail past the storage bays. The forks of the crane are mounted on a platform which is driven vertically up and down the mast, the forks being laterally extendible into the storage bays. When a warehouse is designed for automatic load handling, means are provided for automatically positioning the mast of the crane horizontally and the platform vertically to select a storage bay. For example, the horizontal positioning system may be of the kind described in our British Pat. No. 1,162,371 and the vertical positioning system may be of the kind described in our British Pat. No. 1,142,001. Each of the aforementioned positioning systems is capable of great accuracy, but special problems arise in automatic warehouses due to inaccuracies in the storage frame. For example, consider the positioning system described in our British Pat. No. 1,162,371. This comprises a conductor with true" and *false" crossovers. For horizontal positioning the conductor is stretched along the floor in front of the storage frame and the true crossovers, which define stopping points, are adjusted so that each is aligned with the center of one of the lowest row of storage bays. If the storage bays were accurately aligned vertically, each true" crossover would be aligned with the center of each storage bay in a column. However, this is rarely the case in practice, due to inaccuracies in the construction of the storage frame. In consequence, although the crane may be correctly aligned with a crossover, it will not necessarily be correctly aligned with the selected storage bay. Similar considerations apply to the vertical positioning system. In this instance the conductor is stretched along the mast of the crane and the stopping points for the platform are represented by tapping points along the conductor, as described in our British Pat. No. 1,142,001. However, due to inaccuracies in the storage frame all the storage bays in a horizontal row will not necessarily be aligned with, the corresponding tap on the vertical conductor. It can therefore be seen that if the aforementioned positioning system were the sole means for controlling the position of the crane, it would be possible for the crane to be correctly positioned with respect to the selected stopping points on the horizontal and vertical conductors, and still be misaligned with respect to the selected storage bay. Moreover, the horizontal misalignment may be so great that the forks of the crane will strike one of the vertical members of the frame instead of passing between two vertical members. If the misalignment is too great in the vertical direction, the following problems arise. If the misalignment is in the upward direction, then when a load is being deposited the forks will not be lowered sufficiently far to deposit the load on the horizontal members, and the load will remain on the forks, while if a load is being picked up, the forks will be too high and will strike the load on horizontal members instead of passing under them. If the misalignment is in the downward direction, then when depositing a load, the load will strike the horizontal members instead of passing above them, and when removing a load the forks will not be raised sufficiently to lift the load from the horizontal members.

lt is accordingly an object of the present invention to provide apparatus for detecting positional errors, especially but not exclusively suitable for the above application, in which one or more of the above problems is substantially overcome.

According to the present invention there is provided apwhich the apparatus is attached in relation to a reference surface having a different reflectivity from the surrounding area,

comprising:

a. an extended light source adapted for producing dispersed illumination of said reference surface,

b. at least two photoelectrically sensitive devices adapted for viewing substantially only normally reflected light from respective discrete areas spaced in one direction and for producing in cooperation with the reference surface output electrical signals representative of the illumination received by said devices,

. means for modulating the light from said source at a frequency which is high relative to mains frequency, and

d. phase-sensitive detecting means responsive to said output electrical signals and adapted for deriving a position error signal having a magnitude dependent on the amplitude of the difference between components of said output signals corresponding to the modulation, and having a polarity dependent on the phase of said difference. In order that the present invention may be fully understood and readily carried into effect it will now be described with reference to the accompanying drawings of which:

FIG. 1 shows an example of apparatus for detecting positional error in accordance with the present invention,

FIG. 2 shows a diagrammatic representation of part of the apparatus shown in FIG. 1, together with a circuit diagram of the means for deriving a position error signal,

FIG. 3 shows a further example of apparatus for detecting positional error in accordance with the present invention, and

FIG. 4 shows a diagrammatic representation of part of the apparatus shown in FIG. 3, together with a circuit diagram of the means for deriving a position error signal.

Referring first to FIG. 1, this shows apparatus for detecting positional errors ofa stacker crane comprising a casing l having longitudinal apertures near the top and bottom of the front face. Contained in these apertures are polished reflectors 2 and 3 with lamps 4 and 5 mounted in front of the reflectors, which may be curved instead of angled as shown. Fluorescent lamps are preferred because of their low power consumption, low heat generation and ability to modulate the light beam at a convenient frequency 2 kHz. Also mounted at the front of casing l are four photocells 6, 7, 8 and 9. As shown, the photocells lie at the corners of a square with horizontal and vertical diagonals. However, in some cases the square may be replaced by a rhombus provided that its diagonals are horizontal and vertical. The casing l is attached to the platform of the crane and may, for example, be mounted under or alongside the forks. A patch of different reflectivity from surrounding areas, indicated by reference 10 in FIG. 2, is provided on the storage frame at each storage bay. Each patch may comprise a square white card, or suitable paint, or suitable label etc., but in this example is a white card. Each patch is so positioned that when the crane is correctly aligned with a storage bay, photocells 6, 7, 8 and 9 view four discrete areas on the edge of the patch. The dotted outlines 11 and 12 indicate the preferred maximum and minimum sizes of the patch. As will become apparent, patches outside these limits will give rise to a dead-band in the positioning system.

It will be appreciated that because of the extended nature of the light source formed by the lamps 4 and 5 the illumination of the patch is of a dispersed nature, and light reflected from the patch is similarly dispersed.

The directivity of each photocell must on this account be arranged to be such that they are substantially insensitive to light received from other than substantially normal reflection, so that they only receive light from respective discrete areas. This gives rise to an important advantage in that it renders the positional error sensing substantially insensitive to variation in the normal distance of the casing 1 from the storage racks.

FIG. 2 shows the circuits associated with photocells 6 and 7. Only one lamp, lamp 4 is shown in FIG. 2. In practice, only one lamp is necessary, but it is convenient to use two lamps as a safeguard against failure of a lamp. If only one lamp is used, it may be positioned at the center of the square formed by the four photocells. The power supply for the lamp 4 is modulated by the output from an oscillator 13 which may, for example, operate at 2 kHz. The modulating means is shown schematically as a modulating transformer 14 but any other form of modulator may be used. The other terminal of lamp 4 is connected to a DC bias source. The modulated light from lamp 4 illuminates the patch when the crane platform is nearly in the correct position and the photocells 6 and 7 view discrete areas of the card or the surrounding surface. The modulated outputs from photocells 6 and 7 are capacitively coupled to the inputs of a differential amplifier 15, a potentiometer 16 being provided for balancing the outputs from the photocells 6 and 7. The output from the differential amplifier is applied to a phase-sensitive detector 17 together with a phasereference signal from oscillator 13. Phase-sensitive detector 17 is provided with two output terminals U and D.

The circuit of FIG. 2 operates as follows. If the two photocells 6 and 7 view equally bright areas, as indicated by patch 11 in FIG. 1, the output signals from the two photocells are also equal, differential amplifier 15 produces no output, and no output appears on either of the output terminals of phase-sensitive detector 17. If however photocells 6 and 7 view unequally bright areas, one cell produces a larger output signal than the other. Differential amplifier 15 then produces as an output a ZkI-IZ. signal whose amplitude is proportional to the difference between the amplitudes of the signals from photocells 6 and 7 and whose phase depends on which of the two photocells produce the larger output. Phase sensitive detector 17 detects whether the output signal from amplifier 15 is in phase with, or in phase opposition to, the reference signal from oscillator 13, and produces an output at either terminal U or terminal D according to the phase relationship. The amplitude of the output signal is proportional to the difference between the outputs from photocells 6 and 7 and is used in any suitable manner to cause the crane platform to be driven upwards (output U) or downwards (output D) until the outputs from the two photocells are equal. As has already been mentioned, by varying the size of the light patch a dead-band can be produced in the positioning system. Preferably, however, the dead-band is adjusted by providing adjustable gain control in differential amplifier 15.

The positioning system shown in FIG. 2 may be used as a fine-positioning system in conjunction with a coarse-positioning system. It is therefore necessary to provide means for detecting when the coarse position control signal approaches zero and for disconnecting the coarse control signal from, and connecting the fine control signal to the servoamplifier which controls the drive means. As such means are well known in coarse/fine-positioning systems, they will not be described. If the coarse control signals are modulated in phase according to the direction of the positional errors, then the switching may be performed prior to the phase detector, used for both the coarse and the fine error signals. Alternatively, the switching may be performed after the phase-sensitive detection.

As the circuits associated with photocells 8 and 9 are identical to those shown in FIG. 2, they need not be described. Lamp 4, oscillator 13 and modulator 14, of coarse, need not be duplicated.

The use ofa modulated light source is an important feature of the invention. In the absence of modulation, the error signals would be susceptible to changes in ambient lighting, which would affect the amplitude of the error signals, and what is more important, might affect the photocells unequally. It will be appreciated that by modulation is meant modulation at other than mains frequency.

In some types of stacker cranes, the crane platform is required to stop at two vertical positions at each storage bay. That is to say, when picking up a load the platform is first positioned below the load or pallet, the forks extended, then the platform raised to a second vertical position to allow the forks to lift the load. Similarly when depositing the load, the platform is first positioned at a high position, the forks extended then the platform lowered to a lower position. When the invention is used with such cranes, it may be necessary to provide a second array of photocells 6', and 7', below the first array. However it is not necessary to duplicate the lamps and their associated circuits, or the differential amplifier and phase-sensitive detectors. Instead, switching means may be provided for selectively connecting amplifier 15 either to cells 6 and 7 or to cells 6' and 7. The program control means for the crane usually provides a signal to indicate whether the crane platform is required to be in its upper or its lower position and this signal may be used to effect the switching. Thus a signal indicating that the platform is to be positioned in its upper position causes the photocells 6, 7, 8 and 9 to be connected to their amplifiers 15, whereas a signal indicating that the platform is to be positioned in its lower position causes the photocells 6', 7', 8 and 9 to be connected to the amplifiers 15.

FIGS. 3 and 4 illustrate a further example of apparatus according to the invention in which just two photocells are used and horizontal positional errors only are detected.

Contained in the aperture is a polished reflector 19 and a lamp 20, similar to those described before. It will be appreciated that more than one lamp may be used for reliability. Also mounted at the front of casing 18 are two photocells mounted behind respective lenses 26 and 27, and shields 23 and 24. The casing 18 is attached to the platform of the crane as before. Referring now to FIG. 4, reference 25 indicates the cross section of a storage frame at a storage bay. Its surface facing the photocells, now shown as references 21 and 22, acts as a reflector to light from lamp 20, which floods the whole of the region between the apparatus and the frame 25, so that when the crane is correctly aligned with the storage bay the photocells 21 and 22 view respective discrete areas at the vertical edges of the frame 25. Shields 23 and 24 serve to isolate the photocells 21 and 22 respectively from transmitted light, while lenses 26 and 27 provide directivity for each photocell. Instead of using a surface of the frame 25 as a reflector, a white card may be used, or suitable paint, or a suitable label etc.

FIG. 4 also shows the circuits associated with photocells 21 and 22. The DC power supply from terminals 32 and 33 for the lamp 20 is modulated by the output from an oscillator 35 which may, for example, operate at 2 kHz., the output ofoscillator 35 being amplified in an amplifier 34. The modulating means is shown schematically as a modulating transformer 31 but any other form of modulator may be used. The modulated light from lamp 20 is reflected by the frame 25 and is received by the photocells 21 and 22. The modulated outputs from photocells 21 and 22 are coupled to the inputs of respective amplifiers 28 and 29, whose outputs are applied to a phasesensitive detector 30, together with a phase-reference signal from oscillator 35. Phase-sensitive detector 30 is provided with three output terminals 36, 37 and 38.

The circuit of FIG. 4 operates in a manner similar to the circuit of FIG. 2. If the reflected light patch falls equally on the two photocells an output signal appears on the output terminal 37, and if they are unequal an output signal appears on either of terminals 38 or 36 causing the crane to be driven either left (say) or right until correctly positioned. The photocells are again only sensitive to light received along a substantially parallel beam and provided that the floodlighting of the frame 25 is so arranged that the intensity ofillumination ofthe frame 25 (or, white card etc.) does not fall off rapidly with increasing distance the system is substantially insensitive to range variation. Furthermore if it had been arranged that the beam of light had been directional and the photocells nondireetional. then great care would have to be taken in the alignment of whatever was used as a reflective target. This is not so in the present invention.

Although this example of the present invention has been described with reference to horizontal alignment it will be appreciated that it is equally applicable to vertical alignment;

this is simply achieved by turning the sensing unit described through 90.

What we claim is:

1. Apparatus for detecting error in the positioning of a member to which the apparatus is attached in relation to a reference surface having a different reflectivity from the surrounding area, comprising:

a. an extended light source adapted for producing dispersed illumination of said reference surface,

b. at least two photoelectrically sensitive devices adapted for viewing substantially only normally reflected light from respective discrete areas spaced in one direction and for producing in cooperation with the reference surface output electrical signals representative of the illumination received by said devices,

c, means for modulating the'light from said source at a frequency which is high relative to mains frequency, and

d. phasesensitive detecting means responsive to said output electrical signals and adapted to derive a position error signal having a magnitude dependent on the amplitude of the difference between components of said output signals corresponding to the modulation, and having a polarity dependent on the phase of said difference. 2. Apparatus according to claim 1 in which said light source comprises one elongated fluorescent lamp.

3. Apparatus according to claim 1 including:

a. two further photoelectrically sensitive devices adapted for viewing substantially only normally reflected light from respective discrete areas on the reference surface spaced in a second direction perpendicular to the first and for producing in cooperation with the reference surface further output electrical signals representative of the illumination received by said further devices, and wherein b. said phase-sensitive detecting means is also responsive to said further output electrical signals and is adapted for deriving a further position error signal having a' magnitude dependent on the difference between components of said further output signals corresponding to the modulation, and having a polarity dependent on the phase of the last-mentioned difference.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent Nor 3,644, 739 Dated February 22, 1972 Inventor(s) Wilkinson et al It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected' as shown below:

on the title page, after [30] Foreign Application Priority Data 30 a o o u q 0 a I 0 o I o I Signed and sealed this 17th day of October 1972.

(SEAL) Attest:

EDWARD I- LFLETCHER JR. ROBERT GOTTSCHALK Commissioner of Patents Attesting Officer USCOMM'DC 60376-3 69 U,S. GOVERNMENT PRINTING OFFICE: 1959 0-356334 FORM PO-105O (10-69) 

1. Apparatus for detecting error in the positioning of a member to which the apparatus is attached in relation to a reference surface having a different reflectivity from the surrounding area, comprising: a. an extended light source adapted for producing dispersed illumination of said reference surface, b. at least two photoelectrically sensitive devices adapted for viewing substantially only normally reflected light from respective discrete areas spaced in one direction and for producing in cooperation with the reference surface output electrical signals representative of the illumination received by said devices, c. means for modulating the light from said source at a frequency which is high relative to mains frequency, and d. phase-sensitive detecting means responsive to said output electrical signals and adapted to derive a position error signal having a magnitude dependent on the amplItude of the difference between components of said output signals corresponding to the modulation, and having a polarity dependent on the phase of said difference.
 2. Apparatus according to claim 1 in which said light source comprises one elongated fluorescent lamp.
 3. Apparatus according to claim 1 including: a. two further photoelectrically sensitive devices adapted for viewing substantially only normally reflected light from respective discrete areas on the reference surface spaced in a second direction perpendicular to the first and for producing in cooperation with the reference surface further output electrical signals representative of the illumination received by said further devices, and wherein b. said phase-sensitive detecting means is also responsive to said further output electrical signals and is adapted for deriving a further position error signal having a magnitude dependent on the difference between components of said further output signals corresponding to the modulation, and having a polarity dependent on the phase of the last-mentioned difference. 